The starting/ending year of a series symbol.
Four alphanumeric characters (may include question marks.)
$a Series start year
series start and end years refer to
the first and last years documents in a series are known to be issued
are not necessarily the same years that the first and last sessions or
$b Series end year
not add the series end year (subfield $b)
unless you are certain that a series symbol is no longer used–for
when a series symbol is continued by another series symbol.
970 $a 1969 $b 1971
970 $a 1985
970 $a 2000
970 $a 2004